As i said in a previous post there is a lot to investigate in ancient painting and mostly in what is called "ars illuminandi".
The reason are interesting. First some facts.
All miniaturists where also alchemist. They had to prepare colors, the paper, the brushes et al. by themselves.
A funny and recurring theme is that of the production of simile gold. Simile gold was used for all the imaging going on till renaissance. It is not astounding if some of the alchemist were also at the search of the real gold formula.
Alchemy had some interesting heuristic methods lost with the advent of the more precise, and somewhat more successful in term of outcome, science. The main source of knowledge came form patient the observation of reality, looking for causal connection of simply some kind of analogy or similitude or even bizarre theory. Reality, the world, was mostly looked at. It was the scientist who changed the game. Instead of looking and contemplating more aggressive ways of testing and representing were found. Science with its experiments and models was based upon the idea that not always what we see is how the things work, instead each sense could be fouled.
Today there are at least three professional categories still cultivating such approach to the world. Tibetan monks, homeopathist and guess who ? photographers. Could photography, with its necessary interaction with reality, be the heir of alchemy as a collection of euristic methods based on looking ?
An interesting, albeit difficult, reading is the "de clarea" a latin fragment form XI century all about miniature and "ars illuminandi" tecniques.
For an idea about alchemy see here for a version (do not know how authorittive) of the "corpus hermeticum" attributed to Ermete Trismegisto (a mithical figure),
I am a bit imprecise here for several reasons. The first is that there is almost no written or provable evidence in the above considerations. Second, as i already said i am working under "artistic" licence, where coherence is the last of the desirable properties in forging new concepts from old ones.